Description of the Prior Art
In gas turbine engines and the like, a turbine operated by burning gases drives a compressor which furnishes air to a combustor. Such turbine engines operate at relatively high temperatures. The capacity of such an engine is limited to a large extent by the ability of the material from which the turbine blades (sometimes referred to herein as "buckets") are made to withstand thermal stresses which develop at such relatively high operating temperatures. The problem may be particularly severe in an industrial gas turbine engine because of the relatively large size of certain engine parts, such as the turbine blades. To enable higher operating temperatures and increased engine efficiency without risking blade failure, hollow, convectively-cooled turbine blades are frequently utilized. Such blades generally have interior passageways which provide flow passages to ensure efficient cooling whereby all the portions of the blades may be maintained at relatively uniform temperature.
It is sometimes desirable, however, to vary the amount of cooling air flow within a turbomachinery blade due to uprated, improved turbine designs. The cooling air is at a premium since it must come from elsewhere within the turbine, and at the same time, it is desirable to pass no more cooling air flow than is necessary. Therefore, it would be advantageous to be able to vary or meter the amount of cooling air flow within a particular blade design to match the needs of the turbine.
U.S. Pat. No. 4,236,870 to Hucul, Jr., et al discloses one method of governing the magnitude of cooling air flow through the turbine blade which permits future modification of the blade cooling system without expensive alteration or blade redesign. The Hucul, Jr., et al patent utilizes a metering plate located at the base of the blade containing one or more metering orifices which communicate with the internal cooling passageways of the blade. However, replacing the metering plate to vary cooling air flow necessarily requires removal of the entire blade, and replacement of the metering plate changes the cooling characteristics throughout the entire blade, not just the tip.
Other turbine blade cooling systems and methods are disclosed in the following examples of the prior art:
______________________________________ Patent No. Inventor Issued ______________________________________ 2,763,427 Lindsey Sept. 22, 1950 3,393,894 Redsell Dec. 5, 1966 3,825,984 Linko et al July 30, 1974 3,867,068 Corsmeier et al Feb. 18, 1975 4,203,706 Hess May 20, 1980 4,236,870 Hucul, Jr. et al Dec. 2, 1980 4,859,141 Maisch et al Aug. 22, 1989 ______________________________________
It has also been attempted to provide partial cooling air blockage, utilizing a sheet metal cover plate which partially blocks through-orifices at the trailing edge tip and in the end of the blade. This approach suffers from the disadvantage that stresses in the weld ligament attaching the trailing edge overhang are higher than desirable.
Another approach has been to reduce the size of the cooling orifices in the casting, where ceramic cores are used for the various orifices. A disadvantage of this approach, however, is a tendency toward breakage in the ceramic core of the trailing edge orifice.
Thus, there exists a need in the art for a low-cost, easily adjustable device and procedure for partially blocking the cooling air at the tip of an industrial gas turbine bucket.